Generally, a hydraulic excavator, typical of automotive construction machines, is largely constituted by a vehicular lower structure which can be driven by a vehicle drive motor, an upper revolving structure swingably mounted on the vehicular lower structure for back and forth revolving motions, and a working mechanism liftably supported at the front of the upper revolving structure. An engine is mounted on the rear portion of a revolving frame on the upper revolving structure for the purpose of driving a hydraulic pump, while a cab, fuel tank and operating oil tank are mounted on front portions of the revolving frame.
In the case of a hydraulic excavator as mentioned above, it has been the general practice to employ a Diesel engine which is known for emissions of a large amount of nitrogen oxides (hereinafter referred to as “NOx” for brevity). In this regard, attempts have been made to depurate NOx emissions by installing a NOx purifying device on a hydraulic excavator as a post-treatment means (e.g., see, for example, Patent Literature 1: Japanese Patent Laid-Open No. 2003-120277).
On the other hand, for example, a NOx purifying device is largely constituted by a urea selective reduction catalyst incorporated into an engine exhaust conduit to get rid of nitrogen oxide in the engine exhaust gas, and a ureal water tank serving as a reservoir of ureal water, and a ureal water injection valve for injecting an aqueous urea solution from the ureal water tank at a position upstream of the urea selecting reduction catalyst (e.g., see, for example, Patent Literature 2: Japanese Patent Laid-Open No. 2003-20936).
In this connection, it is known that coagulation occurs to an aqueous urea solution in the ureal water tank when ambient temperature drops approximately to −10° C. or lower. In this regard, in Patent Literature 2, a ureal water tank is located in the vicinity of operational heat sources like an engine, hydraulic pump, control valve, hydraulic motor and operating oil tank which generate heat during operation, for the purpose of preventing coagulation of the aqueous urea solution.
However, in Patent Literature 2 mentioned above, an engine, hydraulic pump, control valve, hydraulic motor and operating oil tank are utilized as heat sources, and a ureal water tank is located in the vicinity of these heat sources. However, it is usually the case that each one of these heat source components including an engine, hydraulic pump, control valve, hydraulic motor and operating oil tank is located in a predetermined position on a revolving frame. That is to say, a position for installation of a ureal water tank is automatically determined by locations of heat source components, with almost no freedom in ureal water tank allocation. Depending upon the position of a ureal water tank relative to nearby components, it may become difficult to carry out a maintenance and service work, for example, a job of refilling an aqueous urea solution into a ureal water tank.
Besides, a ureal water tank needs to be kept free of coagulation of the aqueous urea solution while a construction machine is in operation. However, as heat sources such as an engine, hydraulic pump, control valve, hydraulic motor and operating oil tank differ from each other in heat release value. Accordingly, depending upon a position of installation, it may become difficult to keep a ureal water tank at a constant temperature level.
Furthermore, in a case where a ureal water tank is located in the vicinity of a fixed engine or a hydraulic pump, the ureal water tank is heated only on the side which faces a heat source. In this case, temperature differences occur between different regions of the tank, making it difficult to heat stably the entire aqueous urea solution in the tank.